Prostaglandins are mediators associated with various symptoms such as pain, fever and inflammation. Especially, prostaglandin E2 (PGE2) is a predominant eicosanoid detected in inflammation conditions. In addition, it is also involved in various physiological and/or pathological conditions such as hyperalgesia, uterine contraction, digestive peristalsis, awakeness, suppression of gastric acid secretion, blood pressure, platelet function, bone metabolism, angiogenesis, cancer invasion and metastasis, and the like.
There are four receptor subtypes, EP1, EP2, EP3 and EP4, which display different pharmacological properties. EP4 receptor belongs to a G protein coupled receptor subfamily, is known as a receptor with seven transmembrane, and plays a significant role for biological events which PGE2 relates to by stimulating cAMP production. From the aspect of pharmacological studies, investigation of compounds with EP4 receptor antagonistic activities has been conducted and selective antagonists have also been known.
PGE2 is a predominant eicosanoid detected in inflammation conditions, and in addition, it is also involved in various physiological and/or pathological conditions such as hyperalgesia, uterine contraction, digestive peristalsis, awakeness, suppression of gastric acid secretion, blood pressure, platelet function, bone metabolism, angiogenesis, cancer metastasis, and the like. Non-patent literature 1 to 3 disclose the character of the prostanoid receptors, relationship thereof with therapy, and selective agonists and antagonists most generally used therefor.
On the other hand, it is reported that PGE2 is highly expressed in the cancer tissue in the different types of cancer, and it is also clarified that PGE2 relates to the development of cancer and disease condition. It is known that PGE2 relates to activation of cell proliferation and inhibition of cell death (apoptosis) and plays an important role in the process of cancer progression and metastasis. But because of a lot of reports varying depending on the difference of statuses such as cancer type, progress process and the like, at this time point, the medical consensus has never obtained on which PGE receptor mediates these various functions of PGE2 associated with cancers.
Regarding the relation between EP4 and cancer, at the cellular level, a report (non-patent literature 4) which shows growth inhibition of the colon cancer cell (HCA-7) by an EP4 receptor antagonist, L-161,982, has been found. Regarding metastasis of cancer, a report (non-patent literature 5) which shows metastasis suppression of breast cancer cells by an EP4 receptor antagonist, AH23848 or ONO-AE3-208, and the like have been found.
Meanwhile, regarding the experiments using animals, it has been reported that an EP4 receptor antagonist, ONO-AE2-227 shows reduced formation of azoxymethane-induced colon aberrant crypt foci and reduced formation of intestinal polyp in APC gene knockout mice (non-patent literature 6).
As stated above, it is known that EP4 relates to development, growth and metastasis of cancer, and in addition, reports which suggest prevention, growth inhibition, and metastasis suppression of cancer by EP4 receptor antagonists are also recognized. But the basis of cancer chemotherapy is “to shrink cancer tissues which have already been formed” and examples of EP4 receptor antagonists which show shrinkage of cancer tissues already formed in animal organs have never been known, and there has been a real need for such compounds.